Since display devices using light-emitting elements have high visibility, are suitable for reduction in thickness, and do not have limitations on viewing angles, they have attracted attention as display devices which can take the place of CRTs (cathode ray tubes) or liquid crystal display devices. Specifically proposed structures of active matrix display devices using light-emitting elements are different depending on manufacturers. However, in general, at least a light emitting element, a transistor (a switching transistor) which controls input of video signals to pixels, and a transistor (a driving transistor) which controls value of current supplied to the light-emitting elements are provided for each pixel.
For example, when all the transistors in pixels have the same conductivity type, it is possible to omit some of steps for fabricating the transistors, for example, a step of adding an impurity element imparting one conductivity type to a semiconductor film. Patent Document 1 discloses a display device in which transistors included in pixels are all n-channel transistors.